Multi-channel stimulators are used in a number of implantable or partially implantable medical devices. Some of these devices include spinal cord stimulators and partially implantable and fully implantable hearing devices.
One challenge with such stimulators is keeping power usage to a minimum to conserve battery life. While increasing battery life may be achieved by extending the size of the battery, that runs counter to the goal of reducing the overall device size which is determined partly by battery size. Conservation of energy in implantable, battery operated devices is an important design goal in order to reduce the overall size of the device. Additionally, given a fixed battery capacity and size, conservation of energy is further desired to prolong the life of the battery.
A cochlear device for restoration of hearing is an exemplary device which uses a multi-channel stimulator. Such a device may be fully implantable or partially implantable. In a partially implantable device, there can be two components, an external component containing the battery and an implantable component which contains additional circuitry for processing the stimulation protocol. The power consumed in this processing circuitry, in addition to the power dissipated through the stimulation leads and electrodes, can be substantial.
It is desirable to improve the efficiency of such a device so that the battery can be recharged with less frequency. Frequent recharging is inconvenient to the user and, moreover, causes the rechargeable battery to reach its end of life more quickly.
A multi-channel spinal cord stimulator for treatment of intractable pain is an exemplary, fully implantable device, wherein the battery is contained inside the device. In this application, prolonging battery life is very important to defer surgery to replace the device.
Conventional multi-channel stimulators can be designed to have a single compliance voltage supply that is common to each channel. A “compliance voltage” is the voltage necessary to drive a desired (e.g. programmed) stimulating current through an electrode, which stimulation current is sufficient to cause excitable tissue to be stimulated at the desired intensity. The compliance voltage varies with the impedance of the electrode-tissue interface and the stimulation threshold of the tissue being stimulated.
Each channel in a multi-channel stimulator has varying compliance voltage requirements because the electrodes interfacing with the body tissue provide varying electrode/tissue impedances. For purposes of discussion, the electrode/tissue impedance, which is a combination of resistance and capacitance will be hereinafter referred to as a simple resistance. Although compliance voltage varies at each channel, in conventional multi-channel stimulators, a common compliance voltage is used for each of the channels. This electrical configuration wastes available battery power since it is unnecessary to have each channel operate at the same compliance voltage. In particular the compliance voltage is set to the highest setting required to satisfy the channel having the highest requirement. The other channels are also set to the same compliance voltage even though these other channels may actually need a smaller maximum compliance voltage.